My first thought is a jar of peanut butter and a jar of vitamins. It'll give you enough calories to keep functioning, but that's about it. The other option is survival bars like Datrex or Mainstay. They don't take up much space. If nothing else, two dozen Snickers Bars in you BOB.
Yummy? No (except for the Snickers). Keep you alive? Yes.
How many days worth of food do you want to have? Three days? Three weeks?
How much can you afford to spend? Can you afford to purchase stuff especially for emergencies and throw it out/replace it when it expires or can you store what you eat/eat what you store?
Do you want food that'll just keep you alive or food you'll actually enjoy eating?
As far as raiding for food goes, think outside the box...what businesses are around you? Do they have employee cafetrias or at least snack vending machines? Do they have breakrooms with coffee? Sugar+creamer+water will give you calories. Do food delivery boys or coffee trucks pass by at certain times? Are there any possibilities for guerilla gardening?
Read books about prisoners of war, depression-era living and other stories of people who had to make it through tough times. You'll glean a lot of tips from them as well as have a better understanding about what people need (or are willing to do) to survive.
Short term, it's all about calories...
-Blast
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